Recent Associations of American Railroads (AAR) and Department of Transportation (DOT) railway tank car regulations for stub sill cars require that projections extending below the tank bottom in excess of one (1) inch must be portected by a skid. It is recommended that the skid have a three unit longitudinal dimension along the bottom of the tank for each one unit of downward projection from the bottom of the tank.
One proposed skid design comprises a conical shaped skid welded to the bottom of the tank, tapering downwardly from the tank bottom at a slope of 3:1 and having a center opening where the lading valve is located for unloading of the lading. However the skid is welded to the tank bottom outboard of the universal flange found on presently used tank cars. This is disadvantageous because the weld to the tank bottom requires stress relieving, and the skid adds weight to the car with no functional benefit to the car other than outlet valve protection.
In U.S. Pat. No. 928,924 (1909) a tank car is disclosed in which castings C and D are mounted in the through center sill A and part of the casting C serves as a valve seat for the lading valve E. Casting C is tapered downwardly and outwardly away from the lading valve toward castings D. The through center sill and castings C and D would inherently provide some protection for the lading valve seat and lading valve closure.
However the through center sill design adds weight to the car in constrast to a stub sill tank car, and the taper on the castings C tends to direct obstructions into the lading valve seat and valve closure, rather than away from the valve seat and valve closure.